1. Field of the Invention
The present invention relates to an apparatus for drilling and in situ mixing to construct soil-cement piles for soil solidification purposes. In particular, the present invention relates to a shear blade for a mixing apparatus that requires less vertical force for drilling and withdrawing the apparatus. The apparatus of the present invention also relates to devices for producing ribbed soil-cement piles.
2. Description of Related Art
There are a variety of methods used in the prior art to increase the ability of the ground to support buildings and other structures. One such conventional method for increasing the support provided by the ground is the construction of piles or columns of soil and cement. These piles can be created by excavating soil, inserting a cylindrical casing, and then filling the casing with a combination of excavated soil and cement. Other methods in the prior art create such piles by in situ mixing of the soil and cement. One type of pile used for soil solidification purposes is an end bearing pile. End bearing piles have a generally cylindrical shape and a length that extends from the surface of the ground downward to bedrock, or to a point where the soil is hard and will not settle significantly. However, end bearing piles are typically quite long and thus expensive to construct. Therefore, their use has been limited to larger multiple level buildings where the ground must be firm and settling is unacceptable.
Friction piles have also been used in the prior art in an attempt to provide a more cost effective means of soil solidification. Friction piles similarly have a generally cylindrical shape, but have a limited length. The load bearing capacity of such friction piles is determined primarily by the friction between the soil and the exterior surface of the pile. One key aspect of friction piles is to provide good surface friction. Friction piles are constructed in soft soil and will allow some settling because they do not rest on bedrock or hardened soil due to their limited length. Therefore, their primary use has been limited to smaller housing structures with one or two levels where downward movement of the pile due to settling of the ground is tolerable.
One problem in the prior art, especially for friction piles, is the bearing capacity provided. Especially in soft soil conditions, the pile will not provide the desired bearing capacity. Therefore, more piles must be constructed to increase the density of piles per square foot, and thus, increase the overall bearing capacity per square foot. However, each additional pile that must be constructed requires significant time and effort. Therefore, there is a need for a system and method that can be used to increase the bearing capacity or the frictional resistance to vertical movement of each pile, and thereby eliminate the need to add more piles to increase the overall bearing capacity.
The prior art provides a variety of conventional drilling devices for drilling into the ground and mixing the soil with grout or additives for soil improvement purposes. A major drawbacks of such existing drilling devices is that they can only be used in very soft soil conditions and for shallow drilling. Soil that is hard prevents the use of these existing drilling devices. For example, in situations where the soil has regions that are very compact and dense, existing drilling devices cannot be used. Such hard soil conditions require that the downward force applied to the drilling apparatus, in particular the shear blade, be increased significantly to overcome the huge resistance applied to the shear blade as the drilling apparatus penetrates downward into the soil. When the compact areas of soil are encountered, it is difficult, if not impossible, to move the drilling apparatus further downward because the ends of the shear blade cannot penetrate the compact areas of soil. While the blades and the other portions of the drilling apparatus can be strengthened to increase their ability to penetrate the soil, the cost and time of such reinforcement of the apparatus is not economically feasible.
Another problem with the prior art drilling systems and very compact soil is the difficulty in controlling the drilling direction. Additional resistance encountered by the drilling device in very compact soil requires that additional downward force be applied to the drilling device. This additional downward force pushes the ends of the shear blade through the hardened soil. However, this additional downward force often causes the shaft to flex or bend. The bending of the shaft in turn causes the auger bit to veer off its original linear path making it very difficult to drill a pile along a straight line in the vertical direction as desired. Moreover, the bending of the shaft increases the likelihood that the shaft will break. Thus, the shear blades of the prior art are particularly problematic for other than normal soft conditions.
Therefore, there is a need for a drilling apparatus that is adaptable to a variety of soil types and that reduces the amount of downward force applied to the apparatus. There is also a need for a drilling apparatus that provides improved control over the drilling direction. Finally, there is a need for an apparatus that can create soil-cement piles with increased surface friction.